This invention relates in general to a facsimile transceiver and in particular it relates to a novel carrier recovery circuit for use in such a transceiver.
Facsimile systems are devices wherein the dark/light variations in a document are sensed by a transmitter having an optical reading means. Signals emanating from the reading means are then amplitude modulated on a carrier and transmitted to a receiver at a remote location. There, the signals are demodulated and the demodulated signals are applied to a writing means which reproduces the dark/light variations on a document.
In the modulation scheme commonly employed, a carrier frequency is mixed with a video signal at the transmitter and the lower sideband of the modulated product is transmitted. At the receiver, a local carrier is used to demodulate the incoming signal. The local carrier is either derived from the incoming signal when the carrier component of that signal is detectable or is generated locally when the incoming signal carrier component is not detectable by a carrier recovery circuit. The demodulated output is the difference frequency (lower sideband) produced by product of the local carrier and the modulated incoming signal.
The derivation and generation of the local carrier at a facsimmile receiver from the modulated incoming signal is a difficult problem. At the transmitter, the product of the carrier frequency and the video signal is transmitted as a vestigial side band signal. In order to conserve band width, the higher frequency side band is normally filtered from the modulated signal leaving only the lower frequency side band for transmission. While this approach conserves band width, it has the undesired effect of also reducing the carrier frequency amplitude by as much as fifty percent, thus making it difficult to detect the transmitted carrier frequency at the receiver and therefore making it difficult to derive a local carrier therefrom. Moreover, when the transmitted carrier is not detectable, it is impossible to derive it from the incoming modulated signal and it must be generated at the same frequency and phase as the last detectable carrier signal. In the transmission scheme commonly employed, dark areas of the document are represented by an amplitude of zero in which case there is little or no carrier transmitted. Also, during times when the transmitted video signal has the highest frequency content resulting from fine scanned document detail, a frequency halving technique is employed which greatly suppresses the transmission of carrier. Thus, after periods of extensive black transmission or fine document detail transmission, the prior art techniques of generating the local carrier have suffered from the difficulty of phase and frequency variation such that when black transmission has ceased and the detection of an incoming carrier signal is resumed, the local carrier derived therefrom is substantially out of phase with that which had been generated at the receiver during the black transmission.